It is possible to simplify radio receivers, and in particular radio receivers equipping radio-communications terminals in mobile radiotelephone networks, by not using an intermediate frequency in the receivers and by going directly from the received radio-frequency signal to a baseband signal. It is then possible to omit the elements of the receiver that involve using an intermediate frequency.
One solution for avoiding use of an intermediate frequency is mentioned in the preamble of Document EP-A-0 474 615. That solution consists of retrieving the wanted signal contained in the received signal whose carrier is a signal of the f=Acos.omega.t+.phi. (type, by using the results of multiplying the received signal by a local cos.omega.t signal, and the results of multiplying said received signal by a local sin.omega.t signal, where .omega. has the same value for the received signal and for both local signals.
The results of those two multiplications correspond respectively to an in-phase signal element I and to a quadrature signal element Q, each of which comprises difference components that are in the vicinity of zero frequency, and sum components that are in the vicinity of twice the frequency of the carrier of the received signal. A low-pass filter makes it possible to retain only the difference components, also referred to as "zero frequency" components, of the I and Q signal elements to obtain the wanted portion of the received signal.
One of the problems that arises with receivers using that technique is that interfering transmissions can create DC component shifts in baseband that degrade the wanted signal obtained at the outlet of a receiver, which wanted signal can be received at a power level that is much lower than the power of the interfering DC component.
That can apply to digital cellular radio networks of the GSM type, when a noise source disturbs reception in a terminal by adding a DC component shift or offset to the wanted signal that the terminal is to receive, in particular if the noise-source operates in bursts, i.e. it transmits on a carrier that it time-shares, as is conventional in time-division multiple access mobile radio networks. In which case, the DC component shift does not necessarily disturb the entire received signal, and it acts differently on the in-phase component I and on the quadrature component Q of the received signal.
To demodulate the received signal correctly, it is necessary to take that problem into account, and in particular to determine the characteristics of the interfering shifts affecting each of the components of the received signal, so as to enable said components to be corrected before they are used.